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Stock-based compensation - Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Employee Stock Option      
Stock-based compensation      
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management $ 9,500,000    
Total unrecognized compensation cost related to non-vested restricted stock awards and performance shares to members of management, period of recognition (in years) 2 years 3 months 18 days    
Restricted Stock Award      
Stock-based compensation      
Windfall Net Of Shortfall $ 500,000    
Restricted Stock Award | Directors      
Stock-based compensation      
Stock granted 0 1,052 25,159
Restricted Stock Expense $ 0 $ 52,000 $ 1,600,000
Incentive award, tax benefit   $ 6,000 $ 200,000
Restricted Stock Units (RSUs) | Executive Officers      
Stock-based compensation      
Stock granted 213,511 152,773 166,648
Restricted Stock Expense $ 7,600,000 $ 7,700,000 $ 6,900,000
Incentive award, tax benefit 1,300,000 $ 1,200,000 $ 1,100,000
Fair Market Value of stock vested at vesting date $ 11,200,000    
Stock-Based Compensation, vesting rights the compensation cost associated with the shares of restricted stock is determined based on a two-prong vesting schedule. The first part is vested ratably over five years commencing at the date of grant (the “graduated vesting portion”) and the second part is vested at termination of employment after attaining 55 years of age and 10 years of service (the “retirement vesting portion”). The graduated vesting portion is accelerated at termination of employment after attaining 55 years of age and 10 years of service. The vesting schedule for restricted shares granted on or after 2014 is as follows, the graduated vesting portion is vested ratably over four years commencing at the date of the grant and the retirement vesting portion is vested at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service. The graduated vesting portion is accelerated at termination of employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service.    
Restricted Stock Units (RSUs) | Executive Officers | Share Vesting On Grant Date      
Stock-based compensation      
Fair value of stocks vested at grant date $ 9,800,000    
Restricted Stock Units (RSUs) | Directors      
Stock-based compensation      
Stock granted 43,866 27,449 0
Restricted Stock Expense $ 1,600,000 $ 1,600,000  
Incentive award, tax benefit 300,000 $ 200,000  
Fair value of stocks vested at grant date $ 1,600,000    
Performance Based Shares | Minimum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 50.00%    
Performance Based Shares | Maximum      
Stock-based compensation      
Vested and expected to vest range based on shareholder return 150.00%    
Performance Based Shares | Executive Officers      
Stock-based compensation      
Stock granted 40,432 65,396 72,414
Restricted Stock Expense $ 2,300,000 $ 4,600,000 $ 5,600,000
Incentive award, tax benefit $ 200,000 $ 300,000 $ 400,000
Stock-Based Compensation, vesting rights The number of shares that will ultimately vest ranges from 50% to a 150% of target based on both market (TSR) and performance (EPS and ROA) conditions. The performance shares vest at the end of the three-year performance cycle. If a participant terminate employment after attaining the earlier of 55 years of age and 10 years of service or 60 years of age and 5 years of service, the performance shares shall continue outstanding and vest at the end of the performance cycle.